Add AGENTS.md entry point for AI coding assistants

Follows the spec-first documentation system introduced in #271.
Provides a minimal pointer file directing AI agents to CONTEXT.md
and the spec/guidelines/ structure.

See: https://github.com/zapstore/zapstore/pull/271

Signed-off-by: alltheseas

AGENTS.md

@@ -0,0 +1,21 @@
+# AI Agents
+
+Start here: [CONTEXT.md](CONTEXT.md)
+
+## Quick Reference
+
+| What | Where |
+|------|-------|
+| Project overview | `CONTEXT.md` |
+| Architecture & patterns | `spec/guidelines/ARCHITECTURE.md` |
+| Non-negotiable rules | `spec/guidelines/INVARIANTS.md` |
+| Quality standards | `spec/guidelines/QUALITY_BAR.md` |
+| Product vision | `spec/guidelines/VISION.md` |
+| Feature specs | `spec/features/` |
+| Active work | `work/` |
+
+## Rules
+
+1. Read `CONTEXT.md` first
+2. Never modify files in `spec/guidelines/`
+3. If a spec is unclear, report it—do not guess